Sylas wastes a passive stack during the travel time of the 2nd part of his E.

[GAMEPLAY] Sylas will sometimes auto-attack nothing after hitting the 2nd part of his E

Firstly, I just noticed while watching a replay, thus I don't know what causes it and how often it happens. I will be on the lookout from now on. When connecting the 2nd part of Sylas' E, during his travel time Sylas auto-attacks without having a target. This wastes a passive stack and results in missed damage. Judging by the video I have attached you need to :

1. Use first part of E
2. Use the 2nd part of E
3. While E is flying, click on a target nearby to auto-attack
4. If the auto-attack animation is about to begin as the chains connect, the auto-attack will come out but miss the target.

This description above is my guess as to how it happens. Again, I will test it more and update the discussion.